This invention relates to an expansion bolt, and more particularly to an expansion roof bolt adapted to hold a pressurizing fluid therein and to expand girthwise upon pressurization of the fluid for anchorage of the bolt in a bore in the roof of a mine for supporting the roof.
Various types of anchoring means have been used for supporting the roof of a mine. A widely used anchoring means is the "point-anchored" expansion bolt system comprising a bolt having a head and a threaded shank, and an expansible shell unit on the shank at its end away from the head comprising a shell having a plurality of leaves and a tapered plug threaded on the shank. To anchor the bolt in a bore in the roof of a mine, the bolt with the expansible shell unit thereon is inserted in the bore and the bolt is turned, thereby moving the tapered plug down along the shank into engagement with the leaves of the shell held on the shank against movement therealong for forcing the leaves to move outwardly into engagement with the roof strata. Continued turning of the bolt moves the head up into pressurized engagement with the roof, and tensions the shank for clamping the roof strata together. Among the disadvantages of this system is that it is not effective in relatively soft roof strata, such as that found in uranium mines, and that, over time, the expansible shell unit creeps down in the bore with a resultant decrease in the clamping force applied to the roof strata by the bolt.
Another widely used anchoring means is the so-called "grouted" roof bolt system, such as shown, for example, in U.S. Pat. No. 3,940,941, involving two-component charges of epoxy resin and a bolt having a nut threaded on an end thereof. To install the bolt in a bore in the roof of a mine, charges of unmixed resin are inserted in the bore, the bolt is partially inserted in the bore and rotated to mix the components of the resin, and the bolt is fully inserted in the bore with the nut held in engagement with the roof until the resin sets. On hardening of the resin, the bolt is bonded in place, and pegs the roof strata together. However, "grouted" bolts are more difficult and more expensive to install than "point-anchored" expansion bolts, and provide insufficient load carrying capacity in roof strata having significant fractures and voids in that the resin extrudes into these openings with a resultant decrease in the strength of the bond between the bolt and the roof. Moreover, there is no inspection technique, in general use, to evaluate the integrity of the bond.
Yet another and more recently developed anchoring means is the "friction rock stabilizer" system, such as shown, for example, in U.S. Pat. Nos. 3,922,867 and 4,012,913, involving a hollow cylindrical body of steel open at its ends and split along a line parallel to its longitudinal axis to enable the body to contract girthwise upon being compressed for enabling its insertion in a bore in the roof of a mine of smaller diameter than the diameter of the tube when uncompressed. As the tube is inserted, the compressive force is released and the tube expands girthwise into pressurized frictional engagement with the surfaces of the roof defining the bore. While the body is effective in stabilizing relatively soft roof strata and in retaining its load carrying capacity upon shifting of the mine roof, its load carrying capacity is relatively limited, being essentially equal to the upward force applied to the lower end of the body to insert it in the bore.